1. Field of the Invention
The present invention concerns an apparatus for protecting an electrical device, having two temperature-dependent switching mechanisms each having at least two temperature-dependent switch conditions and at least two external terminals.
2. Related Prior Art
An apparatus of this kind is known from DE-A-42 05 699.
The known apparatus comprises two independent temperature-dependent switches which each have a separate electrically conductive lower housing part. Each lower housing part receives a bimetallic switching mechanism and serves as a first external terminal. The lower housing part is closed off by a cover on which the second external terminal of the respective switch is provided.
The two switches are arranged next to or above one another, a PTC heating block being provided in each case between or below the lower housing parts, resulting in a series circuit made up of the two switches and the heating resistor located therebetween. The two switches and the heating resistor are insulated externally by means of an insulating tube or an outer housing; only the two external terminals provided on the respective cover, and an external terminal constituted by a lower housing part, are accessible from outside.
The switch arranged between the first and the second external terminal is normally closed. The other switch is normally open, the series circuit made up of this switch and the heating resistor being located between the second and third external terminal. The normally closed and normally open switches each have the same switchover temperature.
The known apparatus is used for overtemperature protection of a load in which, in the event of malfunction, there flows a residual current so small that it is not sufficient for the usual self-hold by means of a heating resistor connected in series with the load. In the case of the known apparatus, the normally closed switch is now placed in series with the load, while the series circuit made up of the normally open switch and heating resistor is parallel to the load. In the event of overtemperature of the load, the normally closed switch decouples the load from the voltage supply, while the normally open switch connects the heating resistor to the voltage supply, so that the heat generated in the heating resistor can be used to hold the normally closed and normally open switches in their respective high-temperature settings.
The known apparatus is thus configured for a very specific purpose, the disadvantage here being that two independent switches are used, so that not only are the total dimensions large, but the overall design outlay is also very high, which is associated with corresponding costs.
DE-AS-1 058 606 discloses a temperature-dependent switch, configured as a true changeover switch, which has a temperature-dependent switching mechanism in the form of a bimetallic snap disk. The switch has an electrically conductive housing in which the bimetallic snap disk is guided peripherally. The housing serves as center terminal of the changeover switch. Depending on the temperature, the bimetallic snap disk is in contact, at its center region, with one of two spring-loaded plungers which are held in a manner insulated with respect to the housing, and each constitute one of the two further external terminals.
Depending on the temperature, the bimetallic snap disk creates an electrically conductive connection between the center terminal and one of the two further external terminals. Because of the insulation required between the housing and the two spring-loaded plungers, the known changeover switch is mechanically very complex and bulky. For a number of applications, moreover, the electrically conductive housing must additionally be insulated externally, which requires assembly time and thus results in costs.
As a result of increasingly strict safety regulations and increasingly demanding safety requirements, two or more temperature-dependent switches are often needed for various switching functions in order to protect, in particular, household devices such as washing machines, coffeemakers, clothes dryer, hair dryers, etc. from overheating and overload current. The individual switches must be designed as normally closed or normally open, depending on the requirement; the various switches often must also have different switching temperatures. The apparatus and the changeover switch described above are not suitable for such universal applications; furthermore, their respective complex configuration and the high costs associated therewith are not in line with today's prevailing cost pressure. The size of the known apparatus also precludes use in today's often very small household appliances.